U.S. patent application number 14/340765 was filed with the patent office on 2016-01-28 for cmos pressure sensor with getter using ti-w wire embedded in membrane.
The applicant listed for this patent is ams International AG. Invention is credited to Willem Besling, Martijn Goossens, Marten Oldsen, Remco Pijnenburg, Peter Steeneken, Casper van der Avoort.
Application Number | 20160025583 14/340765 |
Document ID | / |
Family ID | 53673961 |
Filed Date | 2016-01-28 |
United States Patent
Application |
20160025583 |
Kind Code |
A1 |
Besling; Willem ; et
al. |
January 28, 2016 |
CMOS PRESSURE SENSOR WITH GETTER USING TI-W WIRE EMBEDDED IN
MEMBRANE
Abstract
Various exemplary embodiments relate to a pressure sensor
including a pressure sensitive membrane suspended over a cavity,
wherein the membrane is secured by a set of anchors to a substrate;
and a getter material embedded in the membrane, wherein the surface
of the getter is in contact with any gas within the cavity, and
wherein two end points of the getter material are attached through
the substrate by anchors capable of conducting through the
substrate an electrical current through the getter material.
Inventors: |
Besling; Willem; (Eindhoven,
NL) ; Goossens; Martijn; (Eindhoven, NL) ;
Steeneken; Peter; (Eindhoven, NL) ; Pijnenburg;
Remco; (Eindhoven, NL) ; Oldsen; Marten;
(Kleve, DE) ; van der Avoort; Casper; (Prinsenhof,
DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ams International AG |
Rapperswill |
|
CH |
|
|
Family ID: |
53673961 |
Appl. No.: |
14/340765 |
Filed: |
July 25, 2014 |
Current U.S.
Class: |
73/724 ;
29/592.1 |
Current CPC
Class: |
G01L 19/04 20130101;
B81C 1/00158 20130101; G01L 9/12 20130101; B81C 1/00285 20130101;
B81B 7/0038 20130101; G01L 9/0073 20130101 |
International
Class: |
G01L 9/12 20060101
G01L009/12 |
Claims
1. A pressure sensor comprising: a pressure sensitive membrane
suspended over a cavity, wherein the membrane is secured by a set
of anchors to a substrate; and a getter material embedded in the
membrane, wherein the surface of the getter is in contact with any
gas within the cavity, and wherein two end points of the getter
material are attached through the substrate by anchors capable of
conducting through the substrate an electrical current through the
getter material.
2. The pressure sensor of claim 1, wherein the getter material
comprises a thin wire.
3. The pressure sensor of claim 2, wherein the wire comprises
titanium (Ti).
4. The pressure sensor of claim 3, wherein the wire further
comprises tungsten (W).
5. The pressure sensor of claim 3, wherein the wire further
comprises Titanium nitride (TiN).
6. The pressure sensor of claim 2, wherein the wire is 0.7 um wide
or less.
7. The pressure sensor of claim 2, wherein the wire is 40 um long
or greater.
8. The pressure sensor of claim 1, further comprising a second
cavity connected to the cavity by a sealed channel, wherein the
membrane is suspended over both cavities and the sealed channel,
and the getter material is embedded in the membrane proximate to
the second cavity.
9. The pressure sensor of claim 8, wherein the sealed channel
protrudes laterally from the membrane and the cavity of the
pressure sensor at one corner.
10. The pressure sensor of claim 8, wherein the sealed channel is
smaller than 1/10th of the lateral width of the membrane.
11. The pressure sensor of claim 1, further comprising an isolation
trench surrounding the getter material.
12. The pressure sensor of claim 1, further comprising two or more
thermally isolating trenches located next to the getter
material.
13. The pressure sensor of claim 1, wherein a set of etch holes in
the membrane are sealed with one of oxide or nitride.
14. A method of manufacturing a pressure sensor, the method
comprising: suspending a pressure sensitive membrane over a cavity,
wherein a getter wire is embedded in the membrane so that the
surface of the getter wire is in contact with any gas within the
cavity, and the getter wire is electrically connected to a current
source; sealing the cavity hermetically by securing the membrane
with anchors, wherein the current source is transmitted through the
anchors; and reducing gas pressure inside the hermetically sealed
cavity by heating the getter wire.
15. The method of claim 14, wherein the step of heating the getter
material comprises running an electrical current from the current
source through the getter material.
16. The method of claim 15, further comprising: delaying heating
the getter material until the pressure sensor is not in use.
17. The method of claim 14, further comprising: determining the
absolute external pressure irrespective of the gas pressure inside
the cavity; and heating the getter material until the reading of
the pressure sensor is identical to the measured absolute
pressure.
18. The method of claim 14, wherein the step of heating the getter
material occurs after a last step of assembly of a device in which
the pressure sensor is embedded.
19. The method of claim 14, wherein the membrane is suspended over
both the cavity, a second cavity, and a sealed channel connecting
the cavity and the second cavity, and the getter wire is embedded
in the membrane proximate to the second cavity.
20. A method of manufacturing a pressure sensor, the method
comprising: depositing a metal wire and at the same time suspending
a pressure sensitive membrane over a cavity, wherein the wire is
embedded in the membrane so that the surface of the wire is in
contact with any gas within the cavity; etching a sacrificial
layer; sealing etch holes in the membrane with one of oxide or
nitride; and reducing gas pressure inside the cavity by heating the
wire, wherein heating the wire comprises transmitting a small
electrical current through the wire.
Description
TECHNICAL FIELD
[0001] Various exemplary embodiments disclosed herein relate
generally to getters used to regulate pressure changes caused by
outgassing in the cavity of a pressure sensitive membrane.
BACKGROUND
[0002] Micro-electromechanical systems (MEMS) pressure sensors rely
on an accurate measurement of the deflection of a suspended
membrane (e.g. silicon or silicon nitride). Typically such sensors
have well-known piezo resistive or capacitive read-outs. See, e.g.,
U.S. Pat. No. 8,256,298 to Suijlen et al., "MEMS pressure sensor."
In order for these sensors to have an accurate reference pressure,
the cavity underneath the membrane must be sealed perfectly from
the environment, which poses strict constraints on the packaging
used to seal the cavity. Conventional arrangements employ an
expensive dual wafer bonding technique to create a hermetically
sealed cavity.
[0003] Pressure sensors with a capacitive read-out have clear
advantages over pressure sensors with conventional piezo resistive
read-out, including very low power consumption and higher
sensitivity. For example, certain pressure sensors employ a thin
suspended silicon nitride (SiN) membrane as a capacitive MEMS
pressure gauge. To achieve the necessary pressure reference, the
cavity underneath the SiN membrane is hermitically closed using a
plasma-enhanced chemical vapor deposition (PECVD) SiN sealing
process. Device performance is largely determined by the physical,
mechanical and structural properties of this film and the thickness
necessary to prevent holes forming in the membrane. Among other
performance factors, film density and composition determine
out-gassing behavior and diffusion barrier properties. Internal
stress and hence the sensitivity of the pressure sensor are
determined by the membrane's thickness and rigidity, which are
related.
[0004] Because a getter film is currently used in the art to cover
the membrane, the getter material is activated once when the
membrane is sealed by placing the whole structure in a heated
environment.
SUMMARY
[0005] In light of the present need for preventing uncontrolled
outgassing and achieve a stable cavity pressure in a MEMS pressure
sensor, a brief summary of various exemplary embodiments is
presented. Some simplifications and omissions may be made in the
following summary, which is intended to highlight and introduce
some aspects of the various exemplary embodiments, but not to limit
the scope of the invention. Detailed descriptions of a preferred
exemplary embodiment adequate to allow those of ordinary skill in
the art to make and use the inventive concepts will follow in later
sections.
[0006] Various exemplary embodiments relate to a pressure sensor
including a pressure sensitive membrane suspended over a cavity,
wherein the membrane is secured by a set of anchors to a substrate;
and a getter material embedded in the membrane, wherein the surface
of the getter is in contact with any gas within the cavity, and
wherein two end points of the getter material are attached through
the substrate by anchors capable of conducting through the
substrate an electrical current through the getter material. In
alternative embodiments, the getter material includes a thin wire,
which in some embodiments includes titanium (Ti), in some
embodiments includes tungsten (W), and in some embodiments further
includes Titanium nitride (TiN). In some embodiments, the wire is
0.7 um wide or less, and in some embodiments, the wire is 40 um
long or greater.
[0007] In various embodiments, the pressure sensor further includes
a second cavity connected to the cavity by a sealed channel,
wherein the membrane is suspended over both cavities and the sealed
channel, and the getter material is embedded in the membrane
proximate to the second cavity. In some embodiments, the sealed
channel protrudes laterally from the membrane and the cavity of the
pressure sensor at one corner. In some embodiments, the sealed
channel is smaller than 1/10th of the lateral width of the
membrane. In some embodiments, the pressure sensor further includes
an isolation trench surrounding the getter material. In some
embodiments, the pressure sensor includes two or more thermally
isolating trenches located next to the getter material. In
alternative embodiments, a set of etch holes in the membrane are
sealed with one of oxide or nitride.
[0008] Various exemplary embodiments relate to a method of
manufacturing a pressure sensor, the method including suspending a
pressure sensitive membrane over a cavity, wherein a getter wire is
embedded in the membrane so that the surface of the getter wire is
in contact with any gas within the cavity, and the getter wire is
electrically connected to a current source; sealing the cavity
hermetically by securing the membrane with anchors, wherein the
current source is transmitted through the anchors; and reducing gas
pressure inside the hermetically sealed cavity by heating the
getter wire. In alternative embodiments, the step of heating the
getter material comprises running an electrical current from the
current source through the getter material.
[0009] In other embodiments, the method includes delaying heating
the getter material until the pressure sensor is not in use. In
further embodiments, the method includes determining the absolute
external pressure irrespective of the gas pressure inside the
cavity; an heating the getter material until the reading of the
pressure sensor is identical to the measured absolute pressure. In
some alternative embodiments, the step of heating the getter
material occurs after a last step of assembly of a device in which
the pressure sensor is embedded. In alternative embodiments, the
membrane is suspended over both the cavity, a second cavity, and a
sealed channel connecting the cavity and the second cavity, and the
getter wire is embedded in the membrane proximate to the second
cavity.
[0010] Various exemplary embodiments relate to a method of
manufacturing a pressure sensor, the method including depositing a
metal wire and at the same time suspending a pressure sensitive
membrane over a cavity, wherein the wire is embedded in the
membrane so that the surface of the wire is in contact with any gas
within the cavity; etching a sacrificial layer; sealing etch holes
in the membrane with one of oxide or nitride; and reducing gas
pressure inside the cavity by heating the wire, wherein heating the
wire includes transmitting a small electrical current through the
wire.
[0011] It should be apparent that, in this manner, various
exemplary embodiments enable achievement of a stable cavity
pressure in a MEMS pressure sensor.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] In order to better understand various exemplary embodiments,
reference is made to the accompanying drawings, wherein:
[0013] FIG. 1 illustrates an exemplary pressure sensitive membrane
with an embedded getter material;
[0014] FIG. 2 illustrates a cross section of the exemplary pressure
sensitive membrane of FIG. 1;
[0015] FIG. 3 illustrates another cross section of the exemplary
pressure sensitive membrane of FIG. 1;
[0016] FIG. 4 illustrates a simulated temperature rise in a
tungsten meander;
[0017] FIG. 5 illustrates an alternative configuration for a
separate cavity;
[0018] FIG. 6 illustrates an exemplary chart demonstrating the
temperature in the center of the wire for 100 mA current and
0.7.times.0.7 cross section at different lengths.
DETAILED DESCRIPTION
[0019] As noted above, pressure sensors with a capacitive read-out
have clear advantages over pressure sensors with conventional piezo
resistive read-out, including very low power consumption and higher
sensitivity. Furthermore, a significant improvement can be achieved
when the pressure sensitive membrane is built directly on top of an
integrated read-out circuit as a single die solution, reducing
parasitic capacitance and therefore, resulting in a better signal
to noise ratio than stand-alone capacitive pressure sensor dies.
Moreover, form factor and packaging are improved due to the
construction of multiple redundant membranes on top of a CMOS
instead of using individual, physically separated pressure sensor
dies. In addition, performance spread is minimized because of
improved matching and calibration at the die level. Also, there is
a significant reduction in environmental disturbances due to
on-chip shielding of for example, electromagnetic fields.
[0020] For all of these reasons, capacitive pressure sensors may be
constructed on top of the final passivation layer of a CMOS
read-out circuit. The pressure sensor technology may make use of
common back-end of line (BEOL) layers for routing and shielding
plates. The device includes a bottom electrode plate and top
electrode plate separated from each other by a cavity and an
isolation/etch stop layer. An essential part of the sensor is the
movable membrane that deflects under a pressure applied from the
outside. A change in pressure is directly correlated to a change in
capacitance between the metal electrode plates.
[0021] In order to create a free-hanging membrane overlying a
cavity, a sacrificial layer is deposited over the bottom electrode
and etch stop layer. During the manufacturing process, the
sacrificial layer can be removed through tiny holes etched in the
membrane using a dry etch method, which avoids sticking. After the
sacrificial etch, a suspended, perforated membrane is formed which
is subsequently sealed with a SiN or SiO2 dielectric film.
[0022] For purposes of calibration and accuracy, most conventional
micro machined pressure sensors use a hermetic membrane that seals
a reference cavity which is at a certain gauge pressure (in an
ideal case the gauge pressure is a vacuum). The external pressure
can be determined because the pressure difference between the
external pressure and the gauge pressure generates a force on the
membrane, which causes the membrane to deflect. This deflection is
then measured by piezo resistive, capacitive or optical sensors.
There are several issues related to this type of pressure sensor
design.
[0023] First, the gas pressure in the reference cavity needs to be
very stable to avoid drift in sensor output drift over time,
meaning the membrane should have a very high level of hermeticity
such that no air or gas can penetrate through the membrane or along
interfaces. Also, the underlying interconnect layers and the
sealing layer should have a very low out-gassing rate, which is
difficult to achieve because PECVD SiN films contain a lot of
hydrogen, which is easily released. However, even though pressure
sensors are extremely sensitive to changes in cavity pressure,
cavity pressure cannot be controlled in standard piezoresistive
pressure sensors, and signal drift caused by outgassing may go
uncorrected.
[0024] Second, as noted above the cavity pressure needs to be as
small as possible, with vacuum being optimal--if the reference
cavity is at or above a certain pressure, gas inside the cavity
will expand with increasing temperature according to Boyle's law
(P*V=n*R*T), which will reduce the pressure difference over the
membrane, rendering the sensor more temperature dependent and less
predictably accurate.
[0025] Although air-tight cavities may be manufactured by using
PECVD SiNx:Hy membranes, this solution also poses similar problems
because pressure may rise in time when the membrane is exposed to
harsh environments (for example, operation at elevated
temperatures, or H.sub.2 diffusion into the cavity from the outside
environment due to H+ generation and recombination with electrons
at the opposite electrode). In addition, membrane properties may
differ in different conditions, and thus lead to inaccurate
measurements, due to hydrogen effusion from the SiNx:Hy capping
membrane, which is sensitive to PECVD deposition conditions and the
resulting SiNx:Hy composition of the film. In particular, the
hydrogen content of a SiNx:Hy membrane depends on the deposition
conditions, with more hydrogen being incorporated if the deposition
temperature is decreased.
[0026] Another complicating factor in membrane manufacturing using
PECVD SiNx:Hy sealing are the post-deposition annealing conditions
(i.e., before closure of the cavity). Differing conditions before
the cavity is completely sealed effect the extent of out-gassing of
hydrogen into the cavity. Because annealing afterwards could
generate a significant pressure build-up in the cavity, it is
important to cure the membrane before the cavity is closed.
[0027] It is possible to control cavity pressure variation caused
by outgassing during manufacturing by applying a non evaporable
getter for environmental gases such as oxygen (O2), nitrogen (N2),
water (H.sub.2O), carbon dioxide (CO2), carbon monoxide (CO), and
hydrogen (H.sub.2) into the sealed cavity.
[0028] Bulk gettering characteristics are heavily dependent on the
amount of active surface area available for reaction with ambient
gases. If the getter is operating at room temperature, when, for
most gases, very limited bulk diffusion takes place, the surface of
the getter eventually becomes saturated, or passivated, and the
bulk getter ceases to scavenge gas.
[0029] As noted above, getter film is currently used in the art to
cover the membrane. Several non evaporable getter materials are
well known in the art. Titanium is the most widely used
non-evaporable getter for UHV applications. For example,
titanium-based non-evaporable getters (NEG) based on the well known
principle that titanium (Ti) very easily forms compounds such as
oxides at room temperature. Hydrogen can also be captured by
adsorption at the grain boundaries. In order to activate Ti as, for
example, an oxygen getter, a pristine, oxide free Ti interface must
be created. However, Ti oxidizes very rapidly and therefore all Ti
that is exposed to air and/or water vapor during typical
manufacturing techniques will be covered with at least a 2-4 nm
thick TiO2 film. Hence, Ti rapidly loses its getter efficiency
during standard CMOS processing. In the art various methods have
been used to protect the Ti from oxidation, such as covering the Ti
with Nickel (Ni) or Gold (Au) metal. In order to activate the Ti
after it has been metal-covered, the film/structure must be heated
to 250.degree. C. so that either the overlying metal/metal oxide
diffuses into the Ti, or the Ti diffuses out, so that pristine
metallic Ti is exposed.
[0030] As noted above the getter film currently used in the art to
cover the membrane is activated once when the membrane is sealed by
placing the whole structure in a heated environment, but this has
at least two disadvantages: it is not repeatable, and exposes both
the sensor and the structure to unnecessary heating that may damage
other components. Because the process is not repeatable after the
membrane is sealed, cavity pressure cannot be controlled in
standard pressure sensors once the sensor is deployed in the field,
leasing to a loss of sensor accuracy.
[0031] MEMS pressure sensors rely on an accurate measurement of the
deflection of a suspended membrane with reference to a known low
pressure, requiring a stable, hermetically sealed cavity underneath
the membrane. To achieve a stable cavity pressure, the application
of a non-evaporable getter may mitigate sensor drift due to
outgassing and/or tiny leaks in the membrane. In light of the
foregoing, it is desirable to control the cavity pressure of a MEMS
sensor by applying a non-evaporable getter for environmental gases
such as O2, N2, H.sub.2O, CO2, CO, and H.sub.2 into the sealed
cavity, and prevent uncontrolled outgassing.
[0032] Ti as a getter material is well known. Ti non-evaporable
getter is usually sublimated from filaments made of Ti alloys (with
Mo or Ta) heated up to 1500.degree. C., the temperature at which
the Ti vapor pressure is about 10.sup.-3 Torr. Titanium films
provide sticking probabilities of 1-5 (10.sup.-2) for H.sub.2 and
0.4-0.6 for CO at room temperature. At room temperature all gases
except H.sub.2 remain on the surface, resulting in a progressive
reduction of pumping speed (surface blocking). However, H.sub.2
diffuses and its pumping speed is not affected by the pumped
amount. The high (>30 kcal/mole) binding energies prevent
desorption of gases adsorbed on Ti at practical temperatures.
Again, H.sub.2 is an exception, since its lower binding energy
(.about.20 kcal/mole) permits desorption by heating. The initial
pumping speed of a Ti sublimation pump may be restored by a further
sublimation process. The total pumping capacity is therefore very
large and depends on the available amount of Ti in the filament.
Although the greatest pressure of a Ti sublimation pump is in
principle unlimited; in practice it may be spoiled by the presence
of rare gases and methane if pumping for these gases is
inadequate.
[0033] The Ti must be activated as or after the membrane is sealed
and the cavity closed. For this purpose activation temperatures are
required that are relatively high (i.e. larger than 400.degree.
C.), which may cause problems. For example, in CMOS processing an
annealing temperature above 400.degree. C. could lead to
degradation of interconnect performance (such as, for example, poor
line resistance distribution and shifts out of the allowed Cpk
range (Cpk is an index (a simple number) which measures how close a
process is running to its specification limits, relative to the
natural variability of the process)). One way to avoid the use of
elevated temperature is to use other metals that are covering the
Ti film during processing. In some applications an Ni- or Au-
coated Ti film may be activated at relative low temperatures (i.e.
>250.degree. C.).
[0034] However, a difficulty is that materials with low activation
points such as Au and or Ni are not allowed and therefore not
available in CMOS BEOL processing. Therefore, in order to activate
the Ti used as a getter, higher temperatures must be applied. One
disadvantage is that this process cannot be carried out if the
pressure sensor is integrated directly on a printed circuit board
(PCB) and/or already used in an application (i.e. included in the
components of a completed mobile phone, implant, tire, etc.),
either of which have a lower melting point than the available
coating film materials. Therefore, it would be desirable to achieve
the maximum possible temperature to activate the getter material in
the field (e.g., already in use in an application), while
protecting the remaining components of the application from melting
or other damage.
[0035] Referring now to the drawings, in which like numerals refer
to like components or steps, there are disclosed broad aspects of
various exemplary embodiments.
[0036] FIG. 1 shows an exemplary pressure sensitive membrane 100
with an embedded getter material 108. As shown in FIG. 1, in order
to avoid the above issues related to a Ti getter film, the getter
material 108 may be embedded as a thin wire 108 in a suspended
membrane 114 overlying a cavity and secured by anchors 102. In some
embodiments, the pressure sensitive membrane 104, 114 may be
suspended over a cavity and deposited in the same process step as
the Ti/W metal wire 108. The etch holes in the membrane 104, 114
may be sealed with oxide or nitride after the sacrificial layer
etch.
[0037] The wire may be embedded so that the surface of the getter
108 is in contact with any gas within the cavity. An advantage of a
Ti/W wire over a nanostructured film is that the getter material
108 may be regenerated and activated after deployment in the field
of use, so that the material may scavenge released residual gases.
The wire may be heated by a small electrical current transmitted
through anchor connections 110, 112, to reach a temperature
sufficient for Ti getter activation. Additionally, embedding the
wire in a membrane 114 enables a higher wire temperature than if
the wire was constructed on the bottom of the cavity, because a
suspended wire 108 will have a higher thermal resistance towards
the substrate heat sink and therefore will reach a higher
temperature at the same power. This arrangement may allow high
temperatures sufficient to activate the Ti getter either by
sublimation of the Ti or by Ti diffusion towards the internal
surface of the cavity, so that the Ti may be regenerated and react
with the gas inside the cavity.
[0038] In one embodiment, the wire may be included in a separate
cavity 118 connected to but removed from the pressure sensitive
membrane 104 itself to ensure that the pressure sensor 100
performance is not affected by variations in heat or membrane
tensility caused by the wire. In such an embodiment, a second
cavity 118 may be in connection with the pressure sensor cavity via
a narrow channel 106 to allow the movement of gasses between the
cavities. In such an embodiment, the membrane material of the
pressure sensitive membrane 104, channel 106, and separate cavity
118 may be continuous.
[0039] In some embodiments of the invention, the wire 108 may
include not only Ti as getter material but also refractory
materials such as, for example, Tungsten (W) and Titanium nitride
(TiN). For example, W is an attractive refractory metal because it
is available in CMOS fabs and starts to melt at a relatively high
temperature of 3440 C, therefore it remains structurally intact
during heating and remains so after sequential heating cycles,
allowing regeneration of the Ti wire multiple times without the
risk of deformation or "wire meltdown." In some embodiments of the
invention, multiple wires may be used, spaced separately or in a
bundle.
[0040] In one embodiment, as shown from above in FIG. 1, and cross
sections of FIG. 1 AA' and BB', FIGS. 2 and 3, respectively, a
sealed channel 106 may protrude laterally from the main suspended
membrane 104 of the pressure sensor at one corner. The volume of
the pressure sensor cavity 204 may be connected via the sealed
channel 106 to the gas volume of the wired cavity 118. In some
embodiments of the invention, an isolation trench 116 may be
located next to the wire so that the center of the wire may conduct
a higher temperature at the same power input without damaging the
surrounding material. As shown in FIGS. 1 and 3, the wire contacts
110, 112, are within isolation trench 116. (Note as indicated in
FIG. 2, the etch holes in membrane 104, 114 in FIG. 2 have been
filled with SiN seal layer 206; the bottom electrode plate 202
under the pressure sensitive membrane 104, 114, and the Si rich SiN
film 208 used as an etch stop, covering an SiO2 interconnect layer
210).
[0041] In various embodiments, the width of the narrow channel 106
may be small compared to the lateral dimensions of the main sensor
membrane 104 to prevent changes in the mechanical properties that
affect the sensor membrane shape. In one typical exemplary
embodiment, where the lateral membrane dimension is 250 um, the
maximum width of the channel 106 may be smaller than 1/10th of the
lateral width i.e. <25 um. For the same reason, to prevent
changes in the mechanical properties that affect the sensor
membrane shape, the narrow channel 106 may be connected to the main
membrane 104 in one of its corners--this will place the channel 106
farther from the main point of membrane pressure--the center--and
reduce the additional number of points of failure of the membrane
104 caused by adding the channel 106. Where the channel 106 or
channels are kept relatively small and are constructed to minimize
interaction with the membrane structure 104, these additional
features do not significantly influence the main membrane 104
properties, and can be ignored for purposes of membrane deflection
and C-P (regression) models.
[0042] In some embodiments, the gas pressure inside of the
hermetically sealed cavity 204 of a pressure sensor 100 may be
reduced by heating a wire 108 containing Ti.
[0043] In one method, a feed-forward program may establish regular
(periodic) heating of the Ti wire 108, which activates gas pumping
or gettering. This reduces the pressure in the cavity 204 to a
level that is lower than necessary for accuracy of the pressure
sensor (for example, lower than 100 pascal (Pa)). In some
embodiments, the pumping/gettering action may be delayed until no
readout of the pressure sensor is required. In an alternative
method, gas pumping or gettering may be combined with a calibration
method in a feed-back arrangement. For example, a calibration step
may determine the absolute external pressure irrespective of the
gas pressure inside the cavity 204, and subsequently, the Ti wire
pump/getter 108 may be activated until the reading of the pressure
sensor is identical to the measured absolute pressure.
[0044] It should be apparent from the foregoing description that
various exemplary embodiments of the invention may be implemented
in hardware and/or firmware. Furthermore, various exemplary
embodiments may be implemented as instructions stored on a
machine-readable storage medium, which may be read and executed by
at least one processor to perform the operations described in
detail herein. A machine-readable storage medium may include any
mechanism for storing information in a form readable by a machine,
such as a personal or laptop computer, a server, or other computing
device. Thus, a machine-readable storage medium may include
read-only memory (ROM), random-access memory (RAM), magnetic disk
storage media, optical storage media, flash-memory devices, and
similar storage media.
[0045] In some embodiments, either or both methods of reducing the
gas pressure may be applied as an initial procedure as a last step
after manufacturing or assembly. In some embodiments, either or
both methods might be repeated at regular intervals and/or as
needed to maintain accuracy during the sensor's lifetime. For
example, the activation time required to regenerate the getter may
be determined by measuring the actual cavity pressure. (See, e.g.,
U.S. Patent Pub. No. 20130233086, "Mems capacitive pressure sensor"
and U.S. Patent Pub. No. 20130118265, "Mems capacitive pressure
sensor, operating method and manufacturing method").
[0046] The effectiveness of gas pumping or gettering partially
depends on the degree of heat applied through the wire. In the
arrangement shown in FIGS. 1-3, the heat conductivity of the heated
wire relative to its environment is mainly determined by the
conduction of heat through the membrane material 114 from the
anchors 110, 112 of the wires 108. In exemplary arrangements where
the wire 108 is embedded in the membrane 114, much higher
temperatures can be achieved for the same power dissipation than if
the wire 108 is attached on the bottom 208 of the cavity 204. For a
single isolated wire with ambient temperature at its ends the
maximum temperature is proportional to the product of voltage,
current and wire length.
[0047] For a single wire 108 that may be heated on-chip, the
available voltage is limited by the chip supply voltage, for
example, a chip may be able to supply 1.8 V. For example, in order
to dissipate 50 mW at 1 V a current of 50 mA and a resistance of 20
Ohm is needed. In such an exemplary embodiment, if the wire 108 has
a cross section of 0.7.times.0.7 um.sup.2 then the length of the
wire 108 would be 196 um to produce an amount of heat sufficient to
produce an acceptably effective pumping or gettering.
[0048] For example, an exemplary membrane stack (FIGS. 2, 3) may
include a combination of silicon nitride, titanium, titanium
nitride, and tungsten. FIG. 4 shows a simulated temperature rise
400 in a tungsten meander. In a tungsten membrane of 100.times.100
um.sup.2 where a meander of 28 wires of 40 um long and a cross
section of 0.7.times.0.7 um 2 is centered in the membrane 114, the
wires of the meander may be electrically isolated from the
surrounding tungsten by SiO2. In an arrangement where the total
electrical resistance of the meander is 114 Ohm, at a current of 25
mA (and a voltage drop of 2.85 V) a temperature of 1400 K 402 is
reached after 30 us, and the power consumption is 70 mW.
[0049] Some applications may require wires of differing length to
accommodate package requirements and/or membrane sizes. In cases
where the wire length might be reduced, a higher current would be
required to achieve the desired heat conduction. A higher current
is more expensive in terms of chip area because the size of the
power transistor needed to switch the current scales relative to
the required maximum current.
[0050] An alternative embodiment to a separate cavity is shown in
FIG. 5. The maximum wire temperature that can be obtained at a
given power input may be determined by two design parameters, both
of which are preferably as high as possible: the power dissipation
of the wire per unit area, and the thermal isolation from the
substrate. As shown, in addition to trench 512, which may be
similar to trench 116 in FIG. 1, thermally isolating trenches 502,
504, 506, and 508 may be located next to the wire 510 so that the
center of the wire may conduct a higher temperature at the same
power input.
[0051] High power dissipation of the wire per unit area may be
achieved by maximizing the electrical resistance per unit area of
the wire 510 (for example, by using the narrowest practical width
for the wire 510), and by minimizing the electrical resistance of
the terminals of the fuse (for example, by making the terminals at
514, 516 as wide as is practical).
[0052] Thermal isolation from the substrate may be achieved by
embedding the wire into the membrane 518 rather than locating it on
the bottom of the cavity, and by maximizing the thermal resistance
between the space surrounding the wire and the heat sink (for
example, the anchors 520 and the substrate). Maximizing thermal
resistance is particularly important for the hottest area of the
wire, typically the center. Additional thermal isolation may be
achieved by adding additional isolation trenches between the fuse
and the surrounding membrane. Heat is mainly conducted away through
the tungsten and to a much lesser extent through the isolating
oxide or nitride. The heat resistance of tungsten is two orders of
magnitude lower than of the sealing layer 206. In embodiments where
the maximum width (i.e. distance between the wire 510 and the metal
part of the membrane) is a small value (0.7 um--wider gaps cannot
be filled without making the layer much thicker), the heat
conductance may be adjusted by the following factors: the shape of
the surrounding tungsten, the location where the surrounding
tungsten connects to the substrate, and how many isolating trenches
are used in proximity to each other.
[0053] As noted above, the effectiveness of gas pumping or
gettering partially depends on the degree of heat applied through
the wire (108, 510), and activation temperatures are required that
are relatively high (i.e. larger than 400.degree. C.). Although
efficient pumping is preferable, there are physical constraints on
the amount of heat that may be applied, even with high power
dissipation and thermal isolation.
[0054] The melting point of SiO2 is 1600.degree. C., the melting
point of tungsten is 3422.degree. C. and the melting point of Ti is
1940.degree. C. The temperature at which the structure of an
associated device may break due to thermo-mechanical stresses will
vary, but generally may be expected to be lower than 1400 K. It is
possible to achieve this temperature with a typical arrangement of
the invention. For example, a current of 100 mA applied to a 40 um
long W wire with a 0.7.times.0.7 um.sup.2 cross-section would
generate a temperature of 2800 K, dissipated power of 40.8 mW, and
voltage of 0.408 V. An exemplary chart demonstrating the
temperature in the center of the wire for 100 mA current and
0.7.times.0.7 cross section at different lengths is shown in FIG.
6.
[0055] If the thermo-mechanical stress exceeds the fracture
strength or yield stress of the materials, cracks will form that
will allow gas to escape and/or enter the previously sealed cavity,
which is undesirable because it will negatively affect sensor
accuracy as explained above. Thermo-mechanical stresses can also
cause delamination of the layers, facilitating the undesired
opening of the cavity 204. Therefore, limits must be placed on the
amount of heat generated in order to avoid structural failure that
may otherwise be caused by activating the getter 108, 510.
[0056] The power needed for on-chip wire heating without structural
failure depends upon the minimum required temperature to regenerate
and activate the Ti, and the wire configuration, for example, the
wire length, the heat losses towards the anchors 102, and the
substrate. Wire temperatures must reach above 450 C because at
those temperatures the species on the surface of the Ti start to
diffuse into the Ti. For example if a TiN layer is used, the Ti
underneath a TiN layer starts to diffuse through the TiN at a
temperature of 450-500 C, therefore the wire must be heated at
least to 450 C to cause the Ti to regenerate. The diffusion leads
to fresh exposure of Ti metal which could then trap water, oxygen,
nitrogen, etc., as discussed above. Further, a wire configuration
may be used such that the application of a relatively large current
of 100 mA would need a wire length of 18 um in order to achieve a
temperature of 800 K (see FIG. 6).
[0057] A person of skill in the art will understand that different
wire configurations are available depending on the layer used and
constraints of the membrane. For example, if the wire is twice as
long for the same current, the voltage will double and the power
dissipation will double as well, resulting in a temperature 4 times
higher (2 2). The degree of heat dissipation increases
exponentially by the degree of length. Therefore, the same
temperature may be achieved while reducing current consumption by
lengthening the wire (108, 510) by a factor of two while keeping
the voltage constant: the current becomes two times smaller and,
consequently, the power becomes two times smaller as well.
Therefore, in another example, to reduce current consumption by a
factor of 10 (for example, from 100 mA to 10 mA) the wire 108, 510
may be made approximately 10 times longer, i.e. in the prior
example, 180 um. All or most lengths appropriate for a
pressure-sensor getter would be feasible for a sensor used in an
application (i.e. included in the components of a completed mobile
phone, implant, tire, etc.).
[0058] There are various advantages of controlling the cavity
pressure in capacitive pressure sensors using a Ti Wire 108, 510 as
shown and described, including: maintaining low cavity pressure;
mitigating signal drift caused by outgassing over time; activating
the getter material repeatedly using an electrical current through
the Ti Wire (as opposed to heating the entire environment, and thus
the entire device, (e.g. mobile phone, implant), once in order to
heat a covering film); and generating heat in the membrane 114 and
not dissipating heat directly into the substrate, allowing the
getter (wire) 108, 510 to reach higher temperatures at lower power.
Additionally, heat conduction from the heated Ti/W wire to its
environment will mainly be determined by the heat conduction of the
membrane material 114 to its anchors 102, made more effective by
the aspect ratio of the membrane/wire. Further, a Ti/W wire 108,
510 can remain structurally intact at elevated temperatures by
using a refractory metal like W in combination with Ti. Also, the
Ti/W wire 108, 510 may constructed in the same process as the
pressure sensitive membrane 104, 114 without using additional
masks.
[0059] According to the foregoing, various exemplary embodiments
provide for a getter arrangement in a pressure-sensitive membrane
that can achieve the maximum possible temperature to activate the
getter material in the field (e.g., already in use in an
application), while protecting the remaining components of the
application from melting or other damage.
[0060] It should be appreciated by those skilled in the art that
any block diagrams herein represent conceptual views of
illustrative circuitry embodying the principals of the invention.
Similarly, it will be appreciated that any flow charts, flow
diagrams, state transition diagrams, pseudo code, and the like
represent various processes which may be substantially represented
in machine readable media and so executed by a computer or
processor, whether or not such computer or processor is explicitly
shown.
[0061] Although the various exemplary embodiments have been
described in detail with particular reference to certain exemplary
aspects thereof, it should be understood that the invention is
capable of other embodiments and its details are capable of
modifications in various obvious respects. As is readily apparent
to those skilled in the art, variations and modifications can be
affected while remaining within the spirit and scope of the
invention. Accordingly, the foregoing disclosure, description, and
figures are for illustrative purposes only and do not in any way
limit the invention, which is defined only by the claims.
* * * * *